Recently, with rapid development of information and communication technology, a society based on ubiquitous information and communication technology has been formed.
In order to connect information and communication apparatuses anywhere and anytime, sensors each having a computer chip having a communication function need to be installed in all social facilities. Accordingly, problems related to supply of power to such apparatuses or sensors have newly arisen. In addition, as portable apparatuses such as mobile phones, Bluetooth handsets and music players such as iPod have rapidly increased, it takes time and effort for a user to charge batteries. As a method for solving such a problem, recently, wireless power transmission technology is attracting considerable attention.
Wireless power transmission or wireless energy transfer technology refers to technology of wirelessly transmitting electric energy from a transmitter to a receiver using the principle of magnetic induction. In the 1800s, electrical motors or transformers using the principle of electromagnetic induction already started to be used and then methods of radiating radio waves or electromagnetic waves such as lasers and transmitting electric energy were also attempted. Commonly used electric toothbrushes or electric razors are charged using the principle of electromagnetic induction.
Up to now, a wireless energy transfer method may be roughly divided into a magnetic induction method, an electromagnetic resonant method and a power transmission method using a short-wavelength radio frequency.
The magnetic induction method refers to technology of using a phenomenon that, when two coils are adjacently placed and current is supplied to one coil, a magnetic flux is generated to generate electromotive force in the other coil, and is commercially available in small apparatuses such as mobile phones. The magnetic induction method may transmit power of a maximum of several kilowatts (kW) and has high efficiency. However, since a maximum transmission distance is 1 cm or less, an apparatus should be generally located to be adjacent to a charger.
The magnetic induction method uses an electric field or a magnetic field instead of electromagnetic waves or current. The magnetic induction method is hardly influenced by an electromagnetic wave and thus is harmless to other electronic apparatuses and humans. In contrast, the magnetic induction method may be used at a limited distance and in a limited space and energy transfer efficiency is slightly low.
The short-wavelength wireless power transmission method—briefly referred to as an RF method—uses a method of directly transmitting and receiving energy in the form of radio waves. This technology is an RF type wireless power transmission method using a rectenna. Rectenna means is a compound word of “antenna” and “rectifier” and means an element for directly converting RF power into direct current (DC) power. That is, the RF method is technology of converting AC radio waves into DC radio waves and using DC radio waves and, recently, research into commercialization thereof has been actively conducted as efficiency is improved.
Wireless power transmission technology may be variously used in IT, railroad and consumer-electronics in addition to the mobile industry.
If a conductor which is not a wireless power receiver—that is, a foreign object (FO)—is present in a wireless charging area, an electromagnetic signal received from a wireless power transmitter may be induced in the FO. For example, the FO may include coins, clips, pins, and ballpoint pens.
If an FO is present between a wireless power receiver and a wireless power transmitter, wireless charging efficiency may be significantly lowered, and the temperatures of the wireless power receiver and the wireless power transmitter may increase due to increase in ambient temperature of the FO. If the FO located in the charging area is not quickly removed, power waste may occur and the wireless power transmitter and the wireless power receiver may be damaged due to overheating.
Even if an FO is not present in an actual charging area, when a wireless power transmitter incorrectly determines that an FO is present in a charging area, charging may be stopped.
Accordingly, accurate detection of the FO on a charging area is becoming an important issue in wireless charging technology.